The invention relates to a method of providing a transparent coating comprising an aluminum oxide layer on a substrate and of providing a stack of layers the lowermost layer of which consists of aluminum oxide.
A method of providing such metal oxide layers, and including oxides other than that of aluminum according to which first a layer of the relevant metal is provided and the metal is then chemically converted into the relevant metal oxide, is known from German Patent Specification No. 757 763. A similar method is known from U.S. Pat. No. 4,190,321 in which the conversion of the metal into the metal oxide is produced electrochemically.
This type of layer or stack of layers, preferably in a thickness of a quarter of the wavelength of the visible light, for example 550 nm, is often used to reduce reflection. Aluminum oxide is frequently used for this purpose. The above-described method in which the conversion of metal into oxide is produced electrolytically, however, has the disadvantage for aluminum that the aluminum cannot be converted completely into the aluminum oxide. On the inside of the layer a zone of the metal is not converted into the oxide and this results in absorption or scattering taking place at the non-converted metal.
This disadvantage in the use of aluminum the presence of the unconverted metal was disclosed in U.S. Pat. No. 4,190,321. According to this Patent this disadvantage can be obviated by keeping the layer, after the electrolytic oxidation, in contact with a dilute solution of sodium chlorite at 80.degree. C. for 10 minutes.